The design and operation of forming tools is particularly challenging in the shaping of large parts at high temperatures. For example, superplastic aluminum and titanium sheet alloys have been formed at temperatures of the order of 500° C. for aluminum and 1100° C. for titanium into one-piece panels or other articles of complex shape. In hot stretch forming, heated blanks of superplastic sheet material are gripped at their edges and placed over the forming surface of a heated tool. One side of the sheet is stretched into compliance with the forming surface, usually by applying gas pressure to the back side of the sheet using a complementary tool.
Presses with heated platens have been used to heat the complementary tools, oven-like, and to move them between open and closed positions. When the press is in its open position a hot finished part is carefully removed and a new hot sheet metal blank inserted. As the press closes the binding surfaces of the tools grip the edges of the blank for gas pressurization and stretch forming. While the blank may be preheated, the tools are heated by the press platens.
Recently, in forming superplastic AA5083 sheet material, internally heated forming tools have been used in unheated conventional hydraulically actuated forming presses. The internally heated forming tool is provided with thermally insulated outside surfaces including its bottom surface (i.e., the surface opposite the forming surface) at which it is attached to an unheated press bed. The heating has been accomplished with electrical resistance heating cartridges embedded in holes bored in the body of the massive cast steel tool. The electrical heater elements are arbitrarily placed near the forming surface for control of the temperature of the tool especially at the forming area. The heater elements have been used in a plurality of separately powered and separately controlled heating zones in order to better control the temperature of the forming surface of the forming tool and the temperature near the forming surface in the gas chamber defining tool. Separate thermocouples are required for each temperature-controlled zone and different zones are often activated at different times in the operation of the tool.
This practice of using many electrical cartridge heaters in many separate electrically powered and controlled heating zones has been very effective in providing reasonably close control of the temperature of the tool forming surface. Such improved temperature control over platen heating has permitted reductions in the time required to hot stretch form automotive inner and outer decklid panels, tailgate panels, and like panels with complex curves and deep recesses. The forming cycle time for successive parts has been markedly reduced, providing increased throughput and better utilization of large, expensive tools and equipment. The insulated, internally heated tools can be preheated outside of the conventional, unheated hydraulic press, and they better maintain forming temperature during prolonged forming operations with the cyclical opening and closing of the press.
However, the use of many separately powered and controlled heating cartridges has proven cumbersome and expensive. Separate temperature sensors (thermocouples) and separate electronic controllers are required for each zone of several heaters. This invention provides a heated forming tool that can be suitably heated with electrical resistance heater elements powered from a single electrical source and controlled using a single temperature measurement as a single zone. It also provides a method of making such an internally heated forming tool. And it provides a method of forming sheet material parts using such a heated tool. These advantages are generally applicable to the forming of materials at elevated temperatures. But they are particularly applicable to the hot stretch forming of sheet metal parts such as automotive body panels using highly formable aluminum sheet metal alloys.